In the past, multiple chemical assays have been performed on biological fluid samples such as whole blood, plasma, or serum. Generally, such testing has been carried out by continuous-flow systems such as those shown in the U.S. Patents to: L. T. Skeggs, U.S. Pat. No. 2,797,149, issued June 25, 1957; L. T. Skeggs-E. C. Whitehead-W. J. Smythe-J. Isreeli-M. H. Pelavin, U.S. Pat. No. 3,241,432, issued Mar. 22, 1966; W. J. Smythe-M. H. Shamos, U.S. Pat. No. 3,479,141, issued Nov. 18, 1969; and M. H. Shamos-W. J. Smythe, U.S. Pat. No. 3,518,009, issued June 30, 1970; all assigned to a common assignee.
Also, chemical testing of ionic analytes has been performed in an automated fashion using thin films of material, such as shown in the U.S. Pat. No. 4,053,381, issued Oct. 11, 1977 to Hamblen et al.
In order to perform blood testing, however, a great number and variety of tests have to be made. This will naturally require many electrochemical cells of different structures and chemistries. There is little savings in time, sample size and monies in performing each test separately. Rapid and cost-effective methods require a simultaneous analysis of all the analytes in a fluid sample. Emphasis must also be directed to reduction of the sample size; preferably to a few drops or less of blood to minimize demands on the subject, e.g., in the case of infants.
A device that suggests an integrated circuit approach for the testing of a variety of blood analytes in a sample is shown in U.S. Pat. No. 4,020,830 issued to C. C. Johnson et al on May 3, 1977. This device features an integrated array of field effect transistors (FETs), each designed as a discrete sensor. While this is a valid approach to the automated testing of blood samples, certain shortcomings are inherent in this technique.
(a) Only ion-selective FETs have been successfully and reliably demonstrated. When designed to measure non-ionic analytes, the FET structure becomes very complex, because an additional electrochemical cell must be placed at the gate electrode of the FET to influence the measured drain current. This measurement, however, requires a constant current source in addition to the cell FET and external reference electrode.
(b) Instability in any complement will naturally cause fluctuations in the drain current, and, hence, errors in the measurement of the analyte. In addition, the proposed enzyme and immuno FETs have polymer layers, where concurrent processes such as adsorption and ionic double layer capacitance changes can effect the electric field at the gate of the FETs. Extraneous electric fields are also produced at the fringes of the gate area. These effects will likewise cause errors in the analysis of the analytes.
(c) The need for an external reference electrode when measuring non-ionic analytes complicates the integration of a FET array.
(d) FETs will only detect a charged molecule, i.e., an ion. Non-charged analytes do not influence the gate voltage in an interference-free manner. Hence, analytes which can be successfully analyzed are limited.
However, the semiconductor fabrication technology is so advanced that very precise miniature devices can be easily and cheaply manufactured. Furthermore, precedence has been established for superior stability, reproducibility and sensitivity. Hence, this invention seeks to combine the best attributes of two technologies (electrochemistry and semiconductors) to achieve integration of sensors without the drawbacks and limitations of the FET approach.
The present invention contemplates the structure and fabrication of a micro-miniaturized, multi-functional, electrochemical, integrated circuit chip or array of improved electrochemical sensors. This circuit chip requires a minimal sample volume to effect the simultaneous analysis of a plurality of analytes in on-site fashion. In addition, immediate analysis will be affordable by use of this circuit chip which can be easily analyzed, or "read out" by a small, hand-held analyzer or computer at the scene of an emergency or at a patient's bedside. As the circuit chip is relatively inexpensive, it may be disposable. Since the sample can be whole blood, sample handling by the user is minimized. Also, as a plurality of analytes can be simultaneously analyzed, requiring only a minimum volume of blood sample, e.g., one drop or less of fluid, the advantages to be gained by the invention are considerable.